Hmmm, don't think you will wear out a die. Brass can't really cut hardened steel and for sure not carbide. Brass is too soft in comparison to the material dies are made of.

Brass thickness and hardness is the reason for the difference in felt resistance when the ball is coming back out of the necks. I have noticed these differences you mention, but it has never caused a problem.

Yes, lock the expander rod lock nut when retracting it and under slight pressure to bring everything into alignment.